Biliary atresia (BA) is a fibro inflammatory disorder that is the leading cause of neonatal cholestasis and the most common indication for liver transplant in the pediatric population. Although epidemiologic data suggest that BA arises from the interplay of genetic risk factors coupled with environmental exposures, the etiology is unknown. Animal models are limited, and the best-known animal model of the disease, the infection of newborn BALB/c mice with rhesus rotavirus (RRV), has provided important information about immunological aspects of the disease but has been less useful for studying the rapidly progressive fibrosis characteristic of the disease. Insight into the pathogenesis of BA comes from the study of a naturally-occurring animal model of the disease. Over the last 40 years, there have been three epidemics of BA in newborn livestock in Australia associated with ingestion of the plant Dysphania glomulifera by pregnant sheep and cows. Clinical and pathological findings from the affected lambs and calves show striking similarities with human BA, in particular marked fibrosis at the time of diagnosis. We hypothesize that determining the causative toxin will enable the identification of general cholangiocyte damage pathways that lead ultimately to BA and fibrosis in vertebrates. We have thus collected and imported D. glomulifera from the Australian pasture affected by the most recent epidemic and have employed a zebrafish bioassay to successively subfractionate the plant and identify active fractions. We have several fractions with a complexity of 2-20 that, remarkably, cause a BA-like pattern of injury (with atresia of the gallbladder and extrahepatic biliary tree) in zebrafish larvae exposed after biliary morphogenesis has occurred, mimicking human BA. The overall goal of this proposal is therefore to identify the biliary toxins in Dysphania, characterize the cholangiocyte damage pathways they induce, and establish new animal models as a means of understanding the pathogenesis of human BA. This will be achieved through three specific aims: 1. to identify D. glomulifera biliary toxins using the in vivo zebrafish bioassay~ 2. To identify molecular markers and genes that regulate extrahepatic biliary injury in zebrafish larvae treated with D. glomulifera toxins~ and 3. To identify and characterize damage pathways induced by D. glomulifera toxins in mammalian models.